It is well known that the thermal expansion rates of pistons and cylinders in internal combustion chambers are different. To compensate for the differences between the thermal expansion rates of pistons and cylinders, pistons are usually machined to have a smaller diameter than the cylinder they are contained in. This difference in diameters, often referred to as "clearance fitting", ensures that as the piston and cylinder expand in high temperatures, there will be some space between the piston and the cylinder walls during operation. In most engines, the clearance between the piston and the cylinder walls is in the range of 0.02 to 0.04 mm.
To minimize the loss of gas pressure in the combustion chamber, the piston is generally provided with piston rings. The clearance between the piston and the cylinder decreases the sealing performance of the piston rings. As a result, there is a loss of pressure in the combustion chamber of the engine and a subsequent loss of engine performance. Furthermore, because there is a space between the piston and the cylinder, the piston is able to oscillate from side to side. This side to side oscillation of the piston reduces engine power, increases engine noise and further decreases the sealing performance of the piston rings.